I have no clue what the rules are. I usually tip the person a couple dollars when I leave the car -- this is usually a Fairmont doorman, who often helps with luggage. If the person is particularly welcoming and stuff, maybe a bit more. And similar when I leave. But it all depends on the service and service attitude, at least for me. The valet parking folks at QEH are very nice (actually, come to think of it, at every Fairmont I've been), so I generally feel good about tipping them a bit more. As for "rules," I really don't know... I figure, it's my money, so I'll use my own rules
